The term “target” is understood to mean an element possessing reflective properties that are different from those of the area in which it is found. These different reflective properties may be due, nonlimitingly, to a different geometry (e.g. ice peak in a flat extent of ice) or a different composition (e.g. metal structure on a body of water).
The reflective properties may be of specular or diffuse type, or any combination thereof.
GNSS reflectometry is an original and opportunistic remote sensing technique that consists in analysing the electromagnetic waves continuously emitted by the sixty satellites of GNSS positioning systems (GALILEO, GPS, GLONASS, etc.), which are captured by an antenna after reflection from the surface of the Earth. These signals interact with the reflecting area and therefore contain information on its properties, which may be observed in the signal received after reflection. Although the feasibility and advantageousness of this method have been amply demonstrated, in particular in aeronautical fields, the implementation of this technique poses a certain number of problems in a spatial context.
The GNSS is therefore used as a radar, and receives the echo after reflection from the surface of the Earth, and deduces, by analysis of the echo, the properties of this surface.
It is an advantageous alternative to radar, at least for low-cost applications, in particular because it is not necessary to pay for the source (the emitter of the radar, with all that it costs in terms of consumption), and it is continuously available (no need to wait for the return of the satellite or of the carrier of the radar).
GNSS reflectometry has been used for a number of years, in an airborne context, for example in agriculture and the surveillance of forests.
However, the link budget is not high enough for implementation of GNSS reflectometry to be simple with conventional low Earth orbit (LEO) satellites, a low Earth orbit generally being considered to be lower than 2000 km.
In the present case, a target may be a ship on the ocean (small area of specular reflection in the middle of an area of diffuse, on the whole, reflection), a portion of a pole not covered with ice (small area of diffuse reflection in the middle of an area of specular, on the whole, reflection).